Images, even when rendered in high quality, appear flat when displayed on a flat monitor. Numerous approaches toward displaying images that appear three-dimensional have been proposed. These approaches fall into two main categories: volumetric and stereoscopic.
Volumetric displays produce three-dimensional imagery by generating a collection of points within a volume that emit, or appear to emit, light. If these points emit light isotropically, the image appears ‘ghosted’ or ‘transparent.’ A typical volumetric display does not create a true three-dimensional light field because the volume elements are not perceived to block each other, and the images therefore do not display occlusion. Volumetric displays have been disclosed by a number of researchers, including Hirsch (U.S. Pat. No. 2,967,905), Ketchpel (U.S. Pat. No. 3,140,415), and Lewis et al. (IEEE Trans. Elec. Dev., 18, No. 9, pp. 724–732, 1971).
The most common form of stereoscopic displays use shuttered or passively polarized eyewear, in which the observer wears eyewear that blocks one of two displayed images from each eye. Examples include passively polarized glasses, and rapidly alternating shuttered glasses (see, for example, U.S. Pat. No. 4,523,226, awarded to Lipton et al.). While this approach has resulted in some success, having being adopted for use by professionals in the fields of molecular modeling and CAD, these methods have not met with widespread acceptance as observers generally do not like to wear equipment over their eyes. This consideration has motivated developments in the field of autostereoscopic displays.
Autostereoscopic displays perform stereo separation of images internally, and do not require an observer to use additional eyewear. A number of researchers have developed displays that present a different image to each eye, so long as the observer remains in a fixed position in space. Most of these are variations on the parallax barrier method, in which a fine vertical grating or lenticular lens array is placed in front of a display screen. When the observer's eyes remain fixed at a particular location in space, each eye can only see one set of display pixels (even or odd) through the grating or lens array. Each set of pixels displays a view of the image, which the human visual system interprets as a three-dimensional image.
Holographic and pseudo-holographic displays output a partial light field, presenting many different views simultaneously. Also, the imagery can be photorealistic, exhibiting occlusion and other viewpoint-dependent effects (e.g., reflection). This approach has the potential to allow many observers to see the same image simultaneously, but of course requires far greater computational ability and bandwidth than is generally required for a two-view stereo display for a single observer. In many cases, these displays generate a two-dimensional light field, providing horizontal, not vertical, parallax.
In U.S. Pat. No. 5,172,251, Benton discloses a display that creates a light field by holographic light wave interference. More recently, Eichenlaub et al. (Proc. SPIE, 3639, p. 110–121, 1999) disclosed a discrete light field display, which produces up to 24 discrete viewing zones, each with a different or pre-stored image. As each of the observer's eyes transitions from one zone to another, the image appears to jump to the next zone. An observer perceives a sense of depth due to stereo disparity when the observer's two eyes are in two different zones.